1. Field of the Invention
The invention relates to an apparatus and method for image enhancement for example in infrared (IR) and photoemission microscopes.
2. Description of the Related Art
A typical infrared/photoemission microscope is shown schematically in FIG. 1 and comprises a sample stage 10, a sample 12 to be images, an objective lens 14 and a focal plane array 16 on which the image of the sample is formed. Focal plane arrays are solid state detectors comprising arrays of detecting elements typically up to 1000xc3x971000. Each of these corresponds to a single pixel. Variations in the performance of these elements relative to each other give rise to pixel to pixel variations described in more detail below. In infrared microscopy the. infrared emissions from the sample are focused on the focal plane array which provides the image of the sample. Photoemission microscopy is based on the phenomenon that certain samples, such as integrated circuits emit electromagnetic radiation when excited which can be focused, and detected to provide an image of the sample. A known microscope of this type is described in U.S. Pat. No. 6,121,616, which is incorporated herein by reference.
A problem exists with images obtained from microscopes such as infrared or photoemission microscopes in the infrared, which frequently contain undesirable artefacts. These arise for various reasons. These reasons include imperfections in the infrared focal plane array on which the image is formed, for example imaging devices such as a charge coupled device (CCD) or image intensifier giving rise to pixel to pixel variation. In addition aberrations in the lenses cause problems, especially as those aberrations can be worse than, or different from those found in the visible part of the spectrum where the lenses are operating outside their optimum or tested wavelength range. Poor illumination of the sample can reduce the image quality and in the case of backside imaging so can poor contrast due to reflections from the back surface and out of focus features on the back surface.
Known image enhancement systems addressing at least some of these problems include use of a flat field image, widely used in astronomy, and use of an xe2x80x9cunsharp maskxe2x80x9d a photographic technique.
The flat field image technique is used to ameliorate pixel to pixel variations in gain in a sample image at the focal plane array. The flat field image is obtained using the same imaging device, to image a uniform grey subject which can be taken from a library of such subjects. The sample image is multiplied by the reciprocal of the flat field image using known techniques cancelling out the pixel variations in the imaging device.
Various problems exist with this technique. The characteristics of the focal plane array may vary with light intensity, wavelength or time such that the flat field image does not represent identical pixel variations. In addition the other factors reducing image quality, as set out above, are not compensated. As can be seen from FIGS. 2a and 2b, whilst the corrected image shown in FIG. 2b compensates for pixel variations, the poor illumination of the sample is not corrected. The poor contrast and edge definition, arising from operation of the lens outside its optimum range and the fact that the image is obtained through several hundred micrometers of silicon, also remain uncorrected.
The xe2x80x9cunsharp maskxe2x80x9d technique relies on creation of an out-of-focus, blurred or xe2x80x9cunsharpxe2x80x9d image from the sample image. As a result the high frequency components of the sample image are removed in the unsharp image. The unsharp image is then subtracted from the photographic image, removing the low frequency components and hence improving sharpness. Various techniques are available to create the unsharp image and subtract it from the sample image, for example using photographic processing at the enlarging stage (rather than when the photograph is taken) or image processing software as will be known to the skilled person and are set out in U.S. Pat. No. 5,001,573, to Sakamoto et al. which is incorporated herein by reference.
A problem with the unsharp mask technique is that the unsharp image is simply the original image that has been defocused using a convolution function, limiting the amount of image information available and in particular meaning that there is no correction of various of the factors listed above such as pixel variation.
It is an object of the invention to mitigate problems associated with known image enhancement techniques.
According to the invention there is provided an apparatus and method as set out in the appended claims.